The use of an enzyme marker in biological systems will be explored further. Peroxidase-labeled compounds such as biologically functional peptides, nucleic acids, steroids, and pharmacologic agents will be made and tested in biological and immunoassay systems. Use of peroxidase-labeled antibodies at the ultrastructural level will be expanded to include various biopsies, cells in culture and micro-organisms. The availability of a marker which is neither radioactive nor toxic will be beneficial to both investigators and the environment.